ST. PETERSBURG, Fla. —Mitt Romney, his friends often say, is a private man in a public world. But with just one month left until the election, he has calculated that to win the presidency he must do what for years he has been loath to: share intimate stories about his life.

So, as the sun set on his rally here Friday night, the Republican nominee, buoyed by his successful turn on the debate stage, for the first time publicly related emotionally powerful anecdotes. Romney told of ministering to the needy in his Mormon church, including a 14-year-old who was dying of leukemia and summoned “Brother Romney” to his bedside. He also spoke of an old friend who ended up a quadriplegic after an accident and came to see Romney recently, the day before he died.

At a waterfront park in downtown St. Petersburg, a crowd of more than 5,500 stood rapt listening to him speak, many with tears welling in their eyes. “Ohhhhh,” they gasped. And with that, the Romney campaign’s all-out effort to reveal the nominee’s character and compassion received a boost: A politician with an aversion to telling personal stories was, at long last, telling them.

“I’ve seen America,” Romney said. “I’ve seen the greatness of the human spirit in my fellow Americans. It’s what gives me confidence in our future, knowing we can rise to the occasion, time and again. I’ve seen it throughout my life.”

Romney’s friends and advisers — as well as his wife, Ann — have long voiced frustration that tales of his charitable deeds have not blossomed into a gentler portrait. The nominee instead has been shackled by a caricature of a stilted, emotionally distant and out-of-touch multimillionaire who lacks the kind of humanity and verve that Americans have come to expect from their presidents. Romney’s likability deficit with President Obama is perhaps his greatest liability.

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The candidate, advisers said, has long considered talk of his good deeds for political gain unseemly and instead focused his message on his managerial competency and economic prescriptions — which is why Friday night’s rally in St. Petersburg marked an evolution.

Earlier in the day, as they traveled aboard the campaign bus in Florida, Romney told his chief strategist, Stuart Stevens, he was ready to get personal. He was feeling confident coming off his performance in the debate and was reunited here with Ann after a couple days apart. In her brief remarks introducing Romney, she seemed to signal what was to come.

“I was so thrilled at the debate for people to see my husband unfiltered, without any negative ads, without any media trying to interpret what he says and what he feels in his heart,” Ann Romney said. “This is a man who cares about the American people. I’ve seen him throughout his life exhibit extraordinary compassion for others.”

Then, a little over 10 minutes into the candidate’s stump speech, Stevens told a reporter to listen carefully — that Romney’s next riff would be new and worth hearing.

Romney talked about his graduate school classmate Billy, who started a successful business but became quadriplegic after an accident. Billy then devoted himself to spinal injury research, Romney said. A few weeks ago, he came to a campaign event in Atlanta.

“It’s not easy for Billy to get around quadriplegic. . . . He can’t move, of course, his arms and his legs, and he can barely speak, and they brought him forward, a big crowd around him, very hot,” Romney said. “I reached down and I put my hand on Billy’s shoulder and I whispered into his ear. I said, ‘Billy, God bless you. I love ya.’ And he whispered right back to me — and I couldn’t quite hear what he said. He tried to speak loud enough for me to hear. He died the next day.”

Romney said he saw the same kind of spirit in a young boy, David Oparowski, a parishioner at the Boston-area Mormon church where Romney served as a volunteer bishop.

David’s parents, Ted, a retired firefighter, and Pat, spoke emotionally at the Republican National Convention in August about how Romney tended to their son when he contracted leukemia at age 14, and how he later delivered his eulogy. “David’s story is part of Mitt’s story,” Ted Oparowski said.

On Friday night in St. Petersburg, Romney talked about David for the first time.

“It was clear he was not going to make it,” Romney said. “I went into his room one night when he was in bed, and he asked me a very difficult question. He said, ‘Mitt, what’s next?’ He called me ‘Brother Romney.’ ‘What’s next?’ And I talked to him about what I believe is next.”

A few days later, when David was in the hospital, he called Romney.

“I was at work and he said, ‘Can you come by?’ And he said, ‘You went to law school, right?’ ”

Romney told him that he had. “He said, ‘Could you come by and write my will for me?’ ” Romney recalled. “So I went to David’s bedside and got a piece of legal paper, made it look very official. And then David proceeded to tell me what he wanted to give his friends. Talked about his fishing rod, and who would get that. He talked about his skateboard, who’d get that. And his rifle — that went to his brother.”

“I’ve seen the character of a young man like David,” Romney continued. “He had his eyes wide open. There’s a saying: Clear eyes, full heart, can’t lose. David couldn’t lose. I loved that young man.”

It just so happens that a banner with the same adage hung over Romney’s holding room backstage at Wednesday night’s debate in Denver. As he walked out to debate the president, Romney looked up and read, “Clear Eyes. Full Hearts. Can’t Lose!”